In many areas of application there is a need to measure surfaces of objects and thus the objects themselves with high accuracy. This applies to the manufacturing industry, in particular, for which high importance is attached to measuring and checking surfaces of workpieces. For these applications there are a number of measuring apparatuses which are designed for specific tasks and are also designated as coordinate measuring apparatuses or machines. These measuring apparatuses measure the surface by establishing a mechanical contact and scanning the surface. Examples thereof include gantry measuring machines, as described, e.g., in DE 43 25 337 or DE 43 25 347. A different system is based on the use of an articulated arm whose measuring sensor arranged at the end of the multipartite arm can be moved along the surface. Generic articulated arms are described for example in U.S. Pat. No. 5,402,582 or EP 1 474 650. Other methods use optical measurement radiation in order to be able to scan surfaces without contact.
One approach known from the prior art is based here on interferometric methods, such as, for example, optical coherence tomography (OCT), such as are described for example in WO 2009/036861 or the European patent application having the application 11171582.7. The distance measuring methods disclosed therein use, for the purpose of measuring surfaces, a frequency-modulated laser beam for providing measurement radiation, which is emitted onto the surface. The measurement radiation backscattered from the surface is received again and serves for interferometrically measuring the distance from a reference point to the surface, i.e. in the so-called z-direction, wherein a measurement arm and a reference arm are used.
In most exemplary embodiments, the surface is scanned by means of a single measurement channel that measures the distance to a point on the surface, wherein the surface is implemented by the entire probe head being moved over the path of measurement points on the surface. For a multiplicity of applications there is a need, however, to scan a relatively large number of points in a distance-measuring fashion simultaneously or in rapid succession, without this necessitating moving the probe head with the optical system in order over each point to be measured.
One approach known from the prior art consists in using one- or two-dimensionally scanning mirrors in order to be able to move the measurement beam over the surface, without this necessitating a movement of the probe head or the entire measuring arrangement. Corresponding realizations with micromechanical elements for interferometric measuring arrangements in the medical field are described for example in J. Sun et al., “MEMS-based endoscopic OCT”, Int. J. of Opt., 2011. The disadvantage of these mirror-based solutions, however, is the still sequential scanning of spatially extensive objects, which reduces the speed that can be realized. An increase is possible only by means of increased scanning rates or by means of the parallelization of the measuring process orthogonally by the use of a plurality of channels.
Therefore, WO 2009/036861 describes probe heads in which the beam path of the measurement radiation is split into two channels, the emission and reception directions of which are oriented with respect to one another, and probe heads in which a plurality of spatially parallel channels are realized. In these approaches, the measurement channels can be used temporarily in parallel or sequentially, wherein either two or more separate measuring arrangements or alternatively a single measuring arrangement with a separation of the channels, e.g. by means of different polarization directions, are or is possible in the case of simultaneous use. Such an embodiment of the probe head allows measurement of edges or steps, for example. However, as a result of the polarization-dependent separation, the construction is complicated and the number of channels that can be realized is limited.
With regard to the underlying measurement principle, for optical coherence tomography very rapidly tunable sources are known, as described e.g. in T. Klein et al., “Megahertz OCT for ultrawide-field retinal imaging with a 1050 nm Fourier domain mode-locked laser”, Opt. Express 19, 3044-3062 (2011), and very fast approaches exhibiting spectral resolution in the Fourier domain with high-speed line CCDs are also known, as described e.g. in Y. K. Tao et al., “High-speed complex conjugate resolved retinal spectral domain optical coherence tomography using sinusoidal phase modulation”, Opt. Lett. 32, 2918 (2007).
However, both methods exhibit a deficiency of available coherence length, such as is required in the field of industrial metrology, e.g. for use in generic coordinate measuring machines or apparatuses.
For methods of optical coherence tomography in the time domain (time domain OCT) with broadband sources, this would require, on account of the short coherence length, additional scanning in the z-direction, i.e. in the surface normal of the surface to be measured, which in turn necessitates additional drives and increased complexity, cf. T. Dresel et al., “Three-dimensional sensing of rough surfaces by coherence radar”, Appl. Opt. 31, 919 (1992).
Methods of optical coherence tomography in the frequency domain (frequency domain OCT), i.e. with spectral resolution, could be developed further in terms of their capability for parallel scanning by the use of an area sensor instead of a line sensor. However, the low frame rates or read-out speeds and the likewise low typical coherence lengths of the order of magnitude of a few millimeters are disadvantageous here.
In this case, the approach of frequency-modulated OCT can also be enhanced into the two- or three-dimensional scanning range by the use of line or area sensors. Since, on account of the required scanning of the interferogram with hundreds of data points, in this case a complete measurement also necessitates a corresponding recording of many hundreds of data fields by means of line or area sensor, such methods are very slow (<100 Hz) in comparison with typical FD-OCT methods, cf. e.g. S. W. Lee et al., “Line-field optical coherence tomography using frequency-sweeping source”, IEEE J. Selec. Top. Quant. Electr. 14, 50 (2008).
One major shared disadvantage here is also the design used for the probe head in free space optics, the data being generated by line or area sensors. This means that it is no longer possible to spatially divide probe head and signal generation and signal processing with a connection by a monomode fiber. Besides the resultant increased complexity of the probe head and the increased mass thereof, in particular the heating of the probe head brought about by the current consumption has an adverse effect. In the field of coordinate measuring technology it is generally advantageous for the components that are moved over the surface to be fashioned as passively as possible, with the result that thermal influences that deform the carrier structure used for movement cannot arise. Moreover, a low mass of the components to be moved, but in particular of the probe head, leads to an improved dynamic range and to smaller acceleration-dependent deformations of the carrying structure.